A cellular communications system projects any number of cells over the earth at diverse locations. A spectrum is allocated in frequency, in time, by coding, or a combination of these, to the cells so that communications taking place in nearby cells use different channels to minimize the chances of interference. On the other hand, communications taking place in cells located far apart may use the same channels, and the large distance between communications in common channels prevents interference. Over a large pattern of cells, a frequency spectrum is reused as much as possible by distributing common channels over the entire pattern so that only far apart cells reuse the same spectrum. An efficient use of spectrum results without interference.
One problem which cellular communications systems address is the transitioning of communications between cells, as well as the selection of an initial cell. Relative movement between end users and cells causes the end users and the communication links directed thereto to move between cells. In order to permit continuous communications in an ongoing call, the system must "handoff" an in-process call when the end user crosses a cell boundary. If a call is not handed-off to a new cell upon leaving an old cell, the call will eventually be lost because the strength of signals over which communications take place would diminish to a point where the system's radio equipment cannot receive the end user's transmissions, or vice versa.
This selection of a new cell is also necessary for servicing cell determination, a process wherein a subscriber unit transitions to a new cell in order to be available to receive or initiate communications.
Conventional cellular communications systems address the transition problem (e.g., handoff and servicing cell determination) by monitoring and comparing signal strength. A currently used channel associated with one cell may be monitored and compared by a subscriber unit in another cell. Communications are then passed off to the cell with the stronger signal.
The conventional transition technique may work adequately when the distances between subscriber units and system transceivers are relatively small, when the speeds of movement between cells and subscriber units are slow, and when transitions are relatively evenly distributed in time. Historically, such conditions existed for conventional terrestrial cellular systems. In such systems cells did not move with respect to the earth, and the movement between cells and subscriber units resulted from subscriber unit movement in accordance with conventional modes of transportation. However, as traditional cells become congested and are sub-divided into micro-cells, transitions occur more frequently.
Also in satellite cellular systems, where radio equipment is located on satellites orbiting the earth in moving orbits, transitions between cells are also frequent, and the conventional transition techniques may be inadequate.
For example, orbiting satellites are located a relatively large distance from subscriber units, often on the order of several hundred kilometers. The smaller this distance, the greater the speed of the satellite relative to a particular position on the earth. Speeds of over 20,000 km/hr are typical. This fast movement relative to a subscriber unit introduces widely and rapidly varying propagation delays and Doppler frequency offsets into signals transmitted between a satellite and a subscriber unit.
As a mobile subscriber moves from one cell to another cell, a transition procedure is employed. The gateway or switching network indicates to the mobile subscriber to change frequencies from a channel which was used in the first cell to a new frequency of a new channel used in the second cell.
Because there are a number of contiguous cells to which a mobile subscriber may travel, a method for determining into which cell the subscriber is to transition is required. If the subscriber is transitioned into an incorrect cell with a new frequency, the mobile subscriber's call will become lost and disconnected, or calls directed to or originated from him will be undeliverable. Such transition decision making is typically accomplished by measuring the amplitude of the signal received from the mobile subscriber. One such terrestrial cellular telephone system is shown in U.S. Pat. No. 4,654,879, issued on Mar. 31, 1987, to S. Goldman.
Modern terrestrial cellular systems, using higher frequencies for communications, improve the detectability of relative motion between subscriber units and transceivers. Motion may be detected by evaluating Doppler values introduced by motion. Also in satellite cellular systems, the cells projected on the earth are much larger than those of the terrestrial systems. In addition, the satellite moves quite rapidly and as a result, subscribers must be transferred from one cell to the next much more often than in terrestrial systems.
Accordingly, it is an advantage of the present invention to provide a method for selecting target cells using Doppler data for servicing cell determination and handoffs in a cellular communication system.